Buddy has the love and sincerity for Fumi-tan, that even the serious doubters are enlightened and desire to follow his lead.
People like him are like the Heroes of olden times. You can't help but follow.
I'm betting the mom got the MC into Fumi and is the origin. I'm also betting that we see the real Fumi in this arc is revealed to Kotaro and it makes the cult more official
Ok so I’m supposing that the mother is a “reverse-love” kind of character as in she already knows and has scouted out the girls. She is just playing “innocent/cute” but then will come in with “DON’T TRY TO SEDUCE MY CUTE SON WITH SUCH WEAK TACTICS” or something like that.